Within a couple of weeks I was as accomplished with my bike as the next perennially scrape-kneed kid. It wasn't until years later that I realized I was actually turning the handlebar slightly left to initiate a right turn. That little tweak on the bar caused me to lean in the opposite direction - the direction I wanted to go.
This information came in handy on several occasions when I rode my motorcycle - and when I made level turns in an airplane. No, you don't turn the yoke right to go left, but an awful lot goes on during the simple task of turning. We don't think about all these things as a matter of routine. We just do them. But like my two-wheeled revelation, knowing what's happening to the airplane can be an advantage.
Although we'll dissect the maneuver to examine it item by item, much of what's going on with the airplane during a level turn occurs simultaneously, so keep that in mind.
Which Way?
Let's say your heading is 270 degrees and you want to turn to 180 degrees using a 30-degree angle of bank. That's easy - just turn the yoke to the left. How far to the left? It depends on how quickly you want to reach your desired bank angle. The more you turn the yoke, the faster the roll rate, but the airplane doesn't arrive at that roll rate instantly. It takes time to build.
The character of this ramp-up in roll rate, or roll acceleration, is the airplane's roll mode, and it varies among different airplanes. If your airplane's roll rate develops slowly, you might want to get it rolling faster, sooner, by turning the yoke farther. Once the airplane achieves the desired roll rate, you'll have to reduce the yoke deflection to keep the roll rate from increasing. This is like accelerating your car from 30 to 55 mph. You can depress the accelerator slightly and wait, or you can floor it until the speed passes through 50 mph, then back off to the position that will maintain 55 mph.
Now that you've established the roll rate you want, you'll have to take it away when the airplane reaches the desired bank angle. The airplane's roll rate will decay the same way it ramped up. If the roll rate gradually diminishes after you center the yoke (return the ailerons to their faired, undeflected position), you'll have to center the yoke well before the airplane reaches the desired bank angle.
Predicting the amount of lead you need can be difficult, so you might decide to capture the bank angle a little more aggressively. Instead of centering the yoke and waiting for the roll rate to diminish, you might keep the yoke deflected longer, then rotate it briefly past center in the opposite direction before centering it.
As the airplane banks, one wing goes down and the other goes up, which changes the angle at which the relative wind strikes the wings. The relative wind approaching the down-going wing shifts downward, and the relative wind approaching the up-going wing shifts upward. Because the wing's lift is perpendicular to the relative wind, the lift of the down-going wing rotates forward and the lift of the up-going wing rotates aft. Both changes tend to yaw the airplane's nose away from the down-going wing, or opposite the direction you're trying to turn.
Usually, this yaw effect is minor, but if the airplane does yaw, another issue arises. With the airplane yawing away from the direction of turn, the inside wing (the one which was going downward) swings forward and the outside wing swings aft relative to each other. That means the inside wing is traveling forward faster than the outside wing, and producing more lift than the outside wing. This lift differential tries to roll the airplane toward the outside wing, which is opposite the direction you want to go.
If you let this roll in the opposite direction happen, the entire process begins again in the other direction. This effect is minor for most flight conditions, and pilots tend to use whatever yoke deflection is necessary to create the desired roll rate and capture the desired bank angle without regard for these secondary effects.
Use Your Feet
To this point we've only looked at the detrimental effects a change of bank angle has on an airplane attempting a level turn. Now we'll look at turn coordination - what it is, why we do it, and what happens if we don't do it correctly.
When you rotate the yoke to begin the turn, the inside aileron deflects up and the outside aileron deflects down. The down-deflected aileron generates additional lift and with it, additional induced drag. The upward-deflected aileron does the opposite - it decreases lift, which reduces induced drag. The difference in drag between the wings causes the airplane's nose to yaw toward the side with more drag. Because the outside wing has more drag, the airplane yaws opposite the desired direction of turn. Most pilots call this adverse yaw.
Pilots eliminate adverse yaw with coordinating rudder - stepping on the rudder pedal in the direction of the turn. Coordinating rudder doesn't remove the differential drag associated with aileron deflection, but it compensates for it by forcing the airplane to yaw toward the turn direction.
As you apply coordinating rudder, the airplane's nose swings toward the turn direction. Now the outside wing is traveling faster relative to the inside wing. This creates differential lift, but this time it's in favor of the roll direction. That's good news, but now you might have to adjust your yoke deflection if you hadn't counted on coordinating rudder to help the roll rate.
The deflected rudder provides the yawing moment the airplane needs to counter adverse yaw, but it also needs a rolling moment. In most airplanes the rudder's center of lift is above the airplane's center of gravity (CG). Because airplanes rotate around their CG, that deflected rudder tries to roll the airplane. With coordinating rudder, the roll you get from deflecting the rudder is in the opposite direction to the roll you want..
Step by step: You turn the yoke left. Adverse yaw attempts to swing the nose right. You counter the adverse yaw with coordinating left pedal. The rudder deflects trailing-edge-left, creating lift to the right. The lift acts above the CG and tries to roll the plane right wing down.
Again, this roll-due-to-rudder-deflection is usually minor thanks to the short vertical moment arm between the rudder's center of lift and the airplane's CG. But again, your response might be another minor yoke deflection adjustment. Typically, these roll augmentations can make predicting exact yoke placement a challenge.
When Sideslip Happens
Now, what if you have an off day and don't apply the correct amount of coordinating rudder? Maybe you under-coordinated; maybe you used too much rudder; maybe the differential drag caught you by surprise. If the airplane yaws out of alignment with the relative wind, for whatever reason, it "sideslips." Sideslip occurs when the relative wind comes from either the right or left of the airplane's nose.
Directionally stable airplanes don't like sideslip. They want to weathervane into the relative wind. When your airplane's directional stability yaws the plane back toward the relative wind, the wings once again move at different speeds relative to each other. The differential lift from this difference in wing speeds gives you another rolling moment.
The direction of this rolling tendency depends on which side the relative wind comes from. Staying with our example of a left turn, let's say you under-coordinated - didn't apply enough left pedal - and that caused some adverse yaw (nose-right). The relative wind now comes from the left, striking the airplane on its left side, so the airplane is in a left sideslip. Directional stability yaws the airplane nose-left, the right wing travels faster, which causes a left rolling tendency. Sounds good, right?
Unfortunately, the sideslip creates a stronger rolling moment called the "dihedral effect." Airplanes with a positive dihedral effect tend to roll away from the sideslip or away from the relative wind. Dihedral effect is a composite of several factors, only one of which is the wing's geometric dihedral. The bottom line is that the wing on the same side of the airplane as the relative wind will generate more lift than the other wing. The differential lift causes a rolling moment away from the sideslip. This effect can be quite strong, particularly at slow speed (high angle of attack).
If the dihedral effect rolls the airplane, it initiates another set of secondary rolling/yawing effects. More differential drag causes additional yaw moments while the differential lift causes additional rolling moments. That requires more yoke and pedal activity on your part.
When you have a bad turn coordination day, these ancillary rolling and yawing effects cause you a lot of extra yoke and rudder work, even if this work seems imperceptible, and you haven't even reached your desired bank angle yet.
Once the directional stability and dihedral effect start yawing and rolling the airplane, they create a Dutch roll. The Dutch roll is a dynamic mode where the airplane yaws and rolls because of its positive directional stability and dihedral effect.
The two motions feed off each other like this: Sideslip causes the airplane to yaw into the relative wind while it rolls away from the relative wind. Often, the airplane yaws past the relative wind, creating a sideslip from the opposite side, and the process repeats. As the bank angle and sideslip change, all the effects of differential drag and lift, and yawing and rolling moments fire off on top of the ones already in play. A Dutch roll can oscillate for several cycles before the airplane's natural stability causes the motion to subside.
Aware that you're having a bad coordination day, you concentrate on making it a better day. To stop the Dutch roll you must eliminate the sideslip, and you do that quite easily with the rudder. Of course, your rudder inputs create a whole new set of effects, but you now know how to handle them because you're having a better coordination day. As the airplane approaches the desired angle of bank, you deftly apply the yoke and pedal inputs necessary to effect a smooth transition to a steady bank angle.
As your heading begins to change, you notice the bank angle is changing slowly. Maybe it's increasing, maybe decreasing. How can this be? You were careful to coordinate your roll and returned the yoke and rudder pedals to their original (neutral) positions. The rudder or ailerons should not be causing any yawing or rolling moments, but the bank angle is changing. It's changing because of spiral stability - the airplane's tendency to remain, or not remain, in an angle of bank.
Several stability characteristics combine to give an airplane its spiral stability characteristics. If the bank angle increases, you'll have to hold a little out-of-turn aileron and vice versa for a lessening bank angle. Alternatively, you could use rudder deflection to exploit the airplane's strong dihedral effect, but you get a slipped or skidded turn, and that's not very comfortable. A little yoke does the trick, and it settles everything down nicely. Enjoy these few seconds, because you repeat the entire process as you begin to roll out on your intended heading.
Coyote Syndrome
Don't worry. Knowing about all the stuff going on every time you make a level turn won't hinder your flying. Not knowing won't turn you into Wile E. Coyote - safely suspended in midair - until you look down. In a practical sense, we all make level turns without contemplating each of these effects individually. We couldn't. In flight, we simply don't have enough time to figure out what caused what.
The important thing is airplane control, and you know how to do that. You know how to coordinate your turns, and you know how to fix a misjudged coordination effort. But knowing all the stuff affecting the airplane during a level turn does help you figure out why your turns are coordinated, why they are not, and how you can improve and perfect your coordination.
Chances are good that you weren't told all this during the briefing for your first flying lesson. Perhaps your instructor didn't want to overburden you with procedures for your initial flight. Perhaps the instructor was wise enough to show you how it's done, saving the details for some later date. Sometimes it's better that way. Just like riding a bike.
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